Transactions must be included in a block to be properly completed. When you send a transaction, it is broadcast to miners. Miners can then optionally include it in their next blocks. Miners will be more inclined to include your transaction if it has a transaction fee.

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I restarted a few more times, then tried to find my transaction(s) on blockchain.info.

Surprise, it's not there.

However, right now, it shows my 2 receiving addresses as having ~0.104 btc, which is the amount I should have had after sending the first 0.1 btc.I can't find my 0.1 btc being sent out anywhere.

Also, on my old wallet, it seems that the faulty amount is 0.05 btc less than 0.104, which means that multibit may have attemped a double spend, and the first got rejected but Multibit didn't recognize it?

I notice in your images that you've got these wallets in a directory called 'Wallets-QT'. If these were originally Bitcoin QT wallets that you've been exporting private keys from and then importing them into MultiBit then please note that this is NOT supported in MultiBit.

It is pretty difficult to move a wallet from Bitcoin Core to MultiBit because there are hidden change addresses so we advise people not to try to do it.If you have done this then your best bet is to use your original Bitcoin Core to transfer your bitcoin out.

export the keys unencrypted, then edit the file and change the dates behind the keys to be earlier than the first usage, you don't need the exact date, if in doubt just change the year and the month to some time before you first used this key. Don't use a time earlier than the genesis block, this could confuse Multibit and lead to strange behavior. Then import that key file into a fresh wallet and let it sync until its 100% and the balances should be correct again.

alternatively (easier and less error prone and more secure because no plain text files are ever created)

* load the wallet with wallet-key-tool (see my signature)* (optional) you can load more than one file to combine multiple wallets, duplicates will be rejected* update the key creation dates from BCI (find the option in the context menu)* fetch the balances from BCI (context menu), this is the authoritative answer to how many BTC you really have* remove unneeded keys* save as new .wallet file, open that new wallet with Multibit* wait for sync complete and everything should be ok again.

(and don't delete your original files, just in case it doesn't work and you still need them later, but I think you know that already)

At the very least, I still don't know why 2 of the same wallets with the same tx history have different balances.

if you compare the tx history in multibit line by line you will probably find that in one of them some tx are missing, probably the oldest ones, so start comparing with the oldest tx to save time. This is because it did not completely sync the block chain. The method I suggested was to force a fresh wallet with these keys to sync from the beginning (or from the earliest time you used these keys onward).

Older versions of Multibit allowed you to manually specify a date from where to sync when resetting the block chain, newer versions are trying to determine this automatically from the time of the earliest transaction in the wallet and I suspect this algorithm is flawed, especially in the presence of imported keys. From looking at the code I got the impression it is trying very very hard to avoid unnecessarily syncing too far back. Therefore my suggestion to make a fresh wallet with imported keys that have explicitly set their creation time early enough to enforce a sync over the needed time, no matter what.

The numbers on blockchain.info should show you how many BTC you really have, these numbers and the tx history shown there can be regarded as authoritative. Your goal is to bring your Multibit wallet into a state where it will show the exact same numbers. Any other versions of the same Multibit wallet showing different numbers can be ignored because its not correctly synced and missing some tx and if it refuses to sync then its broken.

Therefore import the keys (with their creation dates set early enough) into a brand new empty wallet and let it sync. Then it will show the exact same balance and same tx history as blockchain.info and these numbers will be authoritative!

If your wallet was incompletely synced all the time you probably thought you had more BTC than you really had. Thats probably also the reason the last tx did not go through, You and Multibit both believed you still owned these coins and tried to spend them but you have spent them long ago already and neither you nor Multibit can remember (because at some earlier time the sync must have been messed up) and the tx was not accepted by the network.

Oddly enough, I remember going onto blockchain.info and checking my balance out of curiosity and seeing that it added up to 0.2..

I don't understand your problem.

Blockchain.info and Multibit (from your screen shots) show the exact same balances and transactions, there is not a single Satoshi difference. You had around 0.2 (even 0.34) around 16 Mar, this was 2 months ago, you have spent half of it since then, its all on blockchain.info and also in Multibit, you can use a pocket calculator and go through the transactions backwards in time to see exactly how much you had at what time.